The present invention relates to waveguides. In particular, the present invention relates to improving the quality of the inner surface of waveguides.
Today's waveguides are required to carry electromagnetic radiation at higher frequencies with higher power levels than ever before. Gyrotrons can currently power 31/2" diameter circular waveguides with 500 kW of power at 140 Ghz, with further increases seen in the future. Earlier 21/2" diameter waveguides can carry 200 kW at 35 kW at 250 GHz, but they cannot achieve today's power and frequency levels simultaneously. Electrical arcing is a crucial limiting factor in the ability to increase the power and frequency levels carried by waveguides.
Anything that causes electrical arcs to form inside a waveguide must be avoided because they travel backwards up the waveguide to the power source. An arc can seriously damage or destroy an expensive piece of equipment such as a gyrotron. In general, an arc will be initiated by a high field concentration around discontinuities, and will then be perpetuated by the electromagnetic field inside the waveguide.
A combination of factors can contribute to arc formation inside of a waveguide. Roughness, gaps, or debris on the waveguide surface, as well as increasing the power, frequency, or duration of the microwave pulse--all of these may result in arcing. Recent trends towards increasing the power, frequency, and pulse duration of the microwaves carried inside a waveguide have therefore necessitated smoother and cleaner waveguide walls.
Inasmuch as waveguides are put together in sections, there is a need to form a smooth transition across the border from one section to another. Additionally, a vacuum seal must be maintained at the junction between sections.
In the past, the flanges at the ends of waveguide sections have been fitted with a counterbore around their inner periphery. When the flanges from adjoining waveguide sections are bolted together a groove around the inside diameter of the waveguide is formed from the counterbores. A circular annealed copper gasket fits into the groove that is supposed to provide vacuum sealing and surface continuity. The prior art groove is provided with two circular, opposing steps having rectangular cross sections that press against the sides of the gasket when the flanges are bolted together to insure a tight vacuum seal. Such an arrangement is described for a rectangular waveguide in U.S. Pat. No. 3,201,725 to Johnson, dated Aug. 17, 1965, and assigned to Varian. Similar arrangements have been used more recently with circular waveguides--these flanges are called Shively Flanges and are also associated with Varian. It was previously thought that the Shively Flange type of seal was metal-to-metal, and that the gasket itself was flush with the waveguide wall.
In fact, it has been found that this type of seal is not altogether satisfactory. Small gaps between the gasket and the walls of the groove are formed with this arrangement, and the gasket was found to have sharp edges protruding slightly into the waveguide interior. Apparently, the undesirable gaps and edges are the result of the way the gasket is deformed by the pressure from the opposing steps in the groove.
Therefore, to handle the increasing drive requirements of waveguides there is a demand for an improved seal having a configuration that eliminates all sharp edges and gaps across the boundary between waveguide sections.